Vibratory feeders for small parts such as screws, nuts, plastic pieces and so on are generally AC powered and electromechanically tuned to either a 60 or 120 cycle per second frequency. For bowl-type feeders, a bowl for the parts which includes a spirally ascending track interiorally or externally about its circumference is mounted on an intermediate portion which rests on a base. The intermediate portion is coupled to an AC power source through a power control and is electromagnetically tuned to vibrate the bowl. Vibration of the bowl causes the parts to move upwardly along the spiral track. These parts then move to a machine feed track where they are temporarily stored for feeding into an assembly machine.
Prior art controls have typically included means for adjusting the desired voltage to be provided to vibratory feeders. One example of a prior art control is described in U.S. Pat. No. 4,456,822, having a power control triac which fires when the voltage on a capacitor exceeds the trigger voltage of a diac coupled to the gate of the triac. This diac/triac control is, of course, susceptible to AC power fluctuations, because the triac will not fire unless the AC power provided raises the voltage on the capacitor above the diac trigger voltage. Thus, AC line power fluctuation, which is typical in an industrial environment, will cause a feeder controlled by one of these prior art diac/triac units to operate erratically. The prior art diac/triac control also suffers the disadvantage of not permitting low voltage levels to be supplied to the feeder, since commercially available diacs require at least a voltage input of above 10 V for triggering.
Still other prior art controls have merely utilized variable transformers for firing the feeder bowl drive, but the output voltage of these controls are also susceptible to AC power fluctuation.
The prior art controls have lacked logic circuits, such as feedback and comparator circuits, by means of which the actual AC voltage provided to the feeder can be monitored and maintained constant. Consequently, the vibratory action of feeders controlled by these prior art controls can be erratic if the AC line voltage fluctuates.